Officials are warning that Russian forces are preparing an offensive in Ukraine's southeast.

While Ukraine has pushed back against Russian troops in recent days – taking back territory, including areas around the capital city of Kyiv – the head of NATO warned that Russia is regrouping and plans to deploy soldiers in eastern and southern Ukraine for a "crucial phase of the war."

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"Moscow is not giving up its ambitions in Ukraine," NATO chief Jens Stoltenberg said Tuesday. "We expect a further push in the eastern and southern Ukraine to try to take the entire Donbas and to create a land bridge" to the Crimea Peninsula, which Russia annexed in 2014. Russia-backed separatists in the Donbas have been fighting Ukrainian troops for the last eight years.

Tanya Nedashkivs'ka, 57, mourns the death of her husband, killed in Bucha, on the outskirts of Kyiv, Ukraine, Monday, April 4, 2022. ( AP Photo/Rodrigo Abd)

A Ukrainian serviceman walks past the Antonov An-225 aircraft destroyed during fighting between Russian and Ukrainian forces, at the Antonov airport in Hostomel, outskirts of Kyiv, Ukraine, Monday, April 4, 2022. (AP Photo/Felipe Dana)

A dog runs by destroyed houses and Russian military vehicles during a recent battle in Bucha close to Kyiv, Ukraine, Monday, Apr. 4, 2022. (AP Photo/Efrem Lukatsky)

Ruslan Mishanin bids farewell to his 9-year-old daughter as the train with his family leaves for Poland, at the train station in Odesa, on Monday, April 4, 2022. ( AP Photo/Petros Giannakouris)

A Ukrainian soldier walks with children passing destroyed cars due to the war against Russia, in Bucha, on the outskirts of Kyiv, Ukraine, Monday, April 4, 2022. ( AP Photo/Rodrigo Abd)

Family and friends attend the funeral of 37-year-old sergeant Kostiv Andrew, who was killed in action, at the Lychakiv cemetery, in Lviv, western Ukraine, Tuesday, April 5, 2022. (AP Photo/Nariman El-Mofty)

Stoltenberg also insisted that the alliance is standing by to help Ukraine with military equipment, a day after President Volodymyr Zelenskyy appealed for more weaponry.

Zelenskyy addressed the United Nations' most powerful body, the Security Council, on Tuesday, after more grisly evidence emerged of civilian massacres.

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The bodies of at least 410 civilians have been found around Kyiv and a "torture chamber" was discovered in Bucha.

Satellite imagery from Maxar Technologies showed many of the bodies had been lying in the streets for weeks. The New York Times first reported on the images showing the dead.

President Biden said Russian President Vladimir Putin should be tried for war crimes.

More than 4 million Ukrainians have fled the country since the assault began and more than 7 million have been displaced inside Ukraine, according to the U.N.

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Russia has rejected allegations of atrocities, saying without evidence that the horrific scenes were faked and Moscow said it would speak about Bucha at the U.N. on Tuesday.

The Associated Press contributed to this report.

Russia preparing offensive in southern, eastern Ukraine, officials say

More than 4M Ukrainians have fled the country since the assault began